This invention relates to color television signal encoding, and more particularly to the field of selectively filtering the chrominance signal in a color television signal encoder.
When the NTSC and PAL television systems were originated, one of the key concepts involved the relationship between chrominance and luminance bandwidths. It was reasoned that since the human eye is more sensitive to luminance detail than it is to chrominance detail, and since most of the detail in the real world, and particularly human faces, is in the luminance domain, it is possible to transmit all of the necessary chrominance detail using a narrower bandwidth than that necessary to transmit all of the luminance detail. Accordingly, it was determined that the chrominance signal could be band-limited, permitting it to share the same spectrum with the luminance signal by being impressed as carrier suppressed amplitude modulation components in phase quadrature upon luminance bearing main carrier and be successfully separated out by demodulation in the decoding process.
Ever since this strategic decision, the designers of color television encoders and decoders have been struggling to overcome the side effects of this bandwidth sharing. Because of the spectral overlap between the luminance and chrominance information, it has proven virtually impossible to perfectly separate the two. The incomplete separation of these two signals results in luminance component contamination in the chrominance signal component. The result of this contamination is called "cross-color", the familiar "rainbow" artifacts. Chrominance component contamination in the luminance signal component is known as "cross-luminance" or "dot-crawl".
While it has proven extremely difficult to totally eliminate these artifacts, a variety of means have been invented to reduce them and limit the situations in which they occur. Accordingly, modern television systems display ever decreasing amounts of these effects as these inventions take their place in the industry. Comb filters are now used widely, both in encoders and decoders, and in both the horizontal dimension (adjacent pixels on the same line) and the vertical dimension (same pixel location of consecutive lines), and even in the third dimension of consecutive frames.
The prior art is now replete with variations that attempt to solve one facet of this problem or another. Various schemes for filtering at the receiver/decoder have been used. For example, U.S. Pat. No. 4,686,561 to Harwood et al for a "Vertical Detail Information Restoration Circuit" discloses comb filtering of the composite color video signal to produce comb filtered chrominance and luminance signals. A baseband color difference signal is produced from the comb filtered chrominance signal and the presence of high frequency information in the baseband color difference signal is detected. The bandwidth of the low frequency portion of the comb filtered chrominance signal which is restored to the comb filtered luminance signal is varied in response to a control signal derived from the detected presence of the high frequency information in the baseband color difference signal.
Another approach is described in U.S. Pat. No. 4,651,196 to Harwood et al for "Adaptive Control of the Chrominance Signal Frequency Response in a Video Signal Processing System". In this approach, the adaptive bandpass filter is controlled to have a narrow bandwidth when significant amounts of relatively high frequency vertical detail information are present and to have a wide bandwidth otherwise.
In U.S. Pat. No. 4,731,660 to Faroudja et al for "Processing Methods Using Adaptive Threshold for Removal of Chroma/Luminance Cross-Talk in Quadrature Modulated Subcarrier Color Television Systems" a variable threshold circuit is controlled by a signal derived as a function of at least one of the luminance diagonal transition level, the chroma level, and the averaged luminance activity in the spectrum in the vicinity of the color subcarrier.
U.S. Pat. No. 4,916,526 also to Faroudja et al for "Bandwidth Control for Chrominance and Luminance Paths in Quadrature Modulated Color Television System" discloses a method for reducing the bandwidth of a chrominance path to reduce cross-color artifacts. The method includes the steps of detecting the level of the chrominance component of the signal; developing a control signal from the detected level; and, controlling the bandwidth of the chrominance path in accordance with the control signal so as to reduce bandwidth only when the chrominance component is at a level below a predetermined threshold. An optional further step is to establish a predetermined minimum threshold value of the chrominance level below which the control signal is not generated.
And, U.S. Pat. No. 4,167,020 to Holmes for "Suppression of Luminance Signal Contamination of Chrominance Signals in a Video Signal Processing System" discloses sensing delayed and undelayed versions of the video signal, wherein the amount of delay corresponds to an interval during which rapid luminance transitions capable of producing high frequency signals within the chrominance passband can occur. The sensing multiplier generates a control signal indicative of the presence of the luminance transitions. The control signal is then utilized for controlling the chrominance signal processing channel.
Another approach to reducing these undesirable artifacts is to prefilter either the chrominance or luminance signals or both in the color television signal encoder prior to modulation and transmission. U.S. Pat. No. 4,849,808 to Rossi for a "System for Filtering Color Television Signals Utilizing Comb Filtering of Luminance with Variable Combing Level at Low Frequencies and Filtering of Chrominance by Comb Filtering Time Multiplexed Color Difference Signals" discloses an approach that uses vertical filtering of both chrominance and luminance signal independently of each other.
U.S. Pat. No. 4,656,502 to Hansdorfer for a "Color Mixing or Interference Reduction Circuit in Color TV Decoding Circuits" discloses an encoding method in which increasing amounts of higher frequency components of the luminance signal are subtracted from the luminance signal in response to an increase in the amount of the luminance signal that is within the chrominance subcarrier bandpass.
None of the foregoing approaches deal very effectively with a particular situation that can arise, particularly now that television pictures no longer contain only human faces and scenery, but increasingly contain the output of graphic generators and special effects devices. When the level of luminance details is very low, the normal approach used, of sacrificing the chroma detail to enhance the luminance detail, results in a very soft, washed out, and blurry picture with little of either type of detail.
What is desired is a chrominance filtering system that reduces chrominance detail when sufficient luminance detail is present, but which limits how much chrominance detail is reduced when insufficient luminance detail is present.